Previously the "astring.to_text" correctly handled bytes or string
types. This commit allows using it to convert anything to text. The
use-case is to turn any object into string in a safe manner, which is
useful to replace "str(...)" calls on python2 which fails with unicode
while working well on py3 as well.

Note it's not necessary to convert all "str(...)" calls as objects that
use "__repr__" are usually safe and "astring.to_text" becomes equivalent
of "str(...)", but for objects (eg. Exception, ...) the
"astring.to_text" is required.
Signed-off-by: NLukáš Doktor <ldoktor@redhat.com>